Electron discharge device and method of manufacture



Jan. 1945- J. s. DONAL, JR 2,366,319

ELECTRON DISCHARGE DEVICE AND METHOD OF MANUFACTURE Original Filed Feb. 17, 1939 3000 VOLT VflOC/T) OF PRIMARY fZfCTRUA/Sa MMM Q mnvm INVENTOR. qiefian ,5. Dana 12, UK

ATT URNEY Patented Jan. 2, 1945 ELECTRON DISCHARGE DEVICE AND METHOD OF MANUFACTURE John S. Donal, Jr., Summit, N. J., asslgnor to Radio Corporation of America, a corporation of Delaware Original application February 17, 1939, Serial No.

1942, Serial No. 447,614

7 Claims. (Cl. 250-164) My invention relates to electron-discharge tubes and particularly to cathode ray tubes of the light valve type having an hermetically sealed window difiering in material from that of the remainder of the tube envelope. This application is a division of my copending application issued July 21, 1942,, Patent No. 2,290,581.

In the art of television certain types of light valves utilizing the electron beam of a cathode ray tube to control the illumination of the picture by a valve action on light from an auxiliary light source have been proposed, but have not been altogether feasible because of the inability of the structure to utilize the control action of the beam to the fullest advantage, so that the effect-of the electron beam in controlling the light from the auxiliary source has been inadequate. Other difficulties have been encountered because the structure scanned by the electron beam and responding thereto would not rapidly return to a condition of equilibrium to be in a condition to be rescanned.

The principal object of my invention is to provide a device of the cathode ray light valve type wherein the response is adequate to satisfy operating conditions in modern television transmitting and receiving systems. It is also an object to provide a device which is responsive to the control action of a cathode ray beam to a greater degree than heretofore. It is a further object to provide an improved television receiving tube and a method of construction wherein the above difficulties with the prior art devices are overcome. It is a still further object to. Provide a tube which may be easily fabricated with respect to scaling of the envelope which is mechanically strong, although portions thereof are of very thin material.

In accordance with my invention a local light source of substantially constant intensity is developed at a television receiving station and the light is utilized to illuminate a screen or target which is scanned by an electron beam to produce electrostatic charges which control the transparency or opacity of a light transmitting or absorbing medium between the target and the screen on which an image is to be developed. Further in accordance with my invention I provide a transparent target of material and method of sealing the target material to the envelope whereby the above objects are obtained. These and still other objects, features, and advantages of my invention will become apparent and will at once suggest themselves to those skilled in the art from the following description taken in Divided and this application June 19,

connection with the accompanying drawing in which- Figure 1 is a diagrammatic view of a cathode ray light valve or tube embodying my invention;

Figure 2 is a cross-sectional view of a portion of the envelope structure of the tube shown in Figure 1;

Figure 3 is a-curve indicating the secondary electron emission characterictics of certain of the electrode structure shown in Figure 1; and

Figure 4 is a view of a modified target electrode which may be used in practicing my invention.

Referring to Figure 1, which illustrates a tube and system set forth and claimed in my original application of which this application is a division, the cathode ray tube or light valve includes a highly evacuated envelope or bulb I of cylindrical shape with a tubular arm or neck section enclosing a conventional electron gun. The cylindrical portion of the bulb I is provided at one en'lthereof with a window 2 of optically uniform material such as glass so that light from a substantially constant intensity light source 3 may be formed into parallel rays of light by the lens system 4 and projected through the cylindrical portion of the bulb l. The opposite end of the cylindrical portion of the tube is provided with a closure, likewise of optically transparent material which will hereinafter be referred to as the target 5 being positioned so as to be scanned by an electron beam originating from the gun structure in the neck of the bulb I.

The electron gun assembly 6 is of the conventional type including an accelerating anode 'l is used in conventional television receiving systems. Adjacent the end of the electron gun nearer the target 5 I provide means for sweeping the electron beam over the target 5. For this purpose I have shown the deflection coils H 1 and V for deflecting the electron beam in mutually perpendicular directions such as in a horizontal and a vertical direction respectively. The

coils are supplied with the proper currents for sweeping the beam over the target 5 by conventional horizontal and vertical sweep oscillators, that supplying the coils H being suitably controlled for keystone compensation as well known in the art since the plane of the target 5- is at an angle with respect to the longitudinal axis of and target 5.

the electron It is understood that conven- Jtional deflection plates may be substituted for either one or both of the deflection coils if desired.

In accordance with my invention the target is composed of high electrical resistance material having the desired quality of being optically transparent and sealed to the bulb I by the use of an intermediate vitreous material In as best shown in Figure 2. I have found it necessary to provide a physically rugged target of material having the desired characteristics but which is nevertheless sufficiently thin to meet the operating requirements of my device. In the preferred modifications of my device I prefer mica as a target material because even when suflicient- 1y thin, such as a few thousandths of an inch, it is sufllciently strong to withstand normal atmospheric pressures. The vitreous material In is preferably of powdered glass having a coeflicient of expansion similar to that of the bulb I Thus if the bulb I is of soft glass such as lead glass, the material Ia is preferably of finely divided powdered lead glass having a melting point considerably lower than that of the glass of the bulb I and such that it is reasonably fluid at moderate temperatures such as 600 C. and having a coefilcient of linear expansion of about 87 l0-"/ C. Thus the coefficient of expansion of the glass envelope and the mica are effectively matched by the intermediate glass. I prefer to form a paste of the low melting point powdered glass with water which is then applied to the edges of the bulb I and the target 5. The two parts are then pressed together, placed in an oven and heated to about 600 C. for several minutes or until the powdered glass is fused whereupon the assembly may be cooled and annealed to reduce strains in the seal. It is preferable to make this seal subsequent to the introduction of electrodes into the cylindrical portion of the envelope as later described. However, this seal should preferably be made prior to the introduction of the electrodes forming the electron gun assembly. In addition to several lead glasses of slightly different composition I have sealed mica to lime glass wherein the vitreous material Ia was also powdered lead glass having acoefilcient of expansion of 92 l0""/ C. Powdered glasses of somewhat different melting point and coeilicients of expansion may be employed although the melting point should be considerably lower than that of the glass of the envelope to which the mica is sealed. The method of heating the seal may be any which accomplishes the desired end of fusing the intermediate vitreous material without dam-aging the other components of the seal. It is necessary only that coeflicients of expansion of the component parts of the seal shall be of such magnitude that rupture will not result. I have constructed vacuum seals between a mica disc 2 inches in diameter and a cylindrical glass bulb using mica as thin as 0.005 of an inch. Cathode ray tubes provided with mica faces constructed in the foregoing manner have never failed of operation from causes attributable to the seal in question and have withstood atmospheric pressure indefinitely.

This type of seal is particularly suitable in tubes having a container or reservoir I3 positioned exterior to the envelope or bulb I and adjacent the target 5, the target 5 preferably forming one wall of the container. In the wall of the reservoir I3 and opposite the target 5 a window aligned with the target 5 and also with the window 2 so that light from the light source I may be projected through the cylindrical section of the bulb I and the reservoir I3 and focused on a distant light intercepting or viewing screen I! by the lens system Ill. The target 5 and window I4 constitute in effect a double wall for the envelope or bulb I having therebetween a liquid II suspending, for example, very small flakes or particles of graphite. The distance between the adjacent surfaces of the target 5 and the window I4 is not critical although it should be small. Preferably this distance should be substantially equal to the diameter of the electron beam from the electron gun. I have made satisfactory tubes of this type where this distance was less than one millimeter. On one surface of the reservoir window I4 preferably the outside surface thereof, I provide a substantially transparent electrically conductive electrode I8, preferably an exceedingly thin fllm of metal, such as gold or platinum, which may be sputtered, condensed from the vapor phase, or otherwise applied as well known to those familiar with the art of making thin conductive films.

Further, in accordance with myinvention, I make the target 5 of such a material that when bombarded by energy such as by a beam of electrons of relatively low velocity the charge produced thereon is negative whereas when bombarded with higher velocity electrons the charge developed thereon is positive in that more secondary electrons are liberated from the target than there are arriving primary electrons, the material also having the property that under still higher velocity electron bombardment fewer secondary electrons are produced than there are arriving electrons. The electron beam produced and focused on the target 5 by the electron gun is modulated in intensity by the control electrode. The intensity of the beam over a period of one vertical sweep of the beam may be utilized to produce a picture produced in a manner well known in the art but instead of the picture being rendered visible by the cooperation of a fluorescent material deposited on the target the electrons produce an electrostatic image of the picture, which electrostatic image varies in intensity of charge from area to area in ac- I4 of glass or other transparent material is axially 1 cordance with the light and shade areas from which the signals applied to the control electrode were derived. The electrostatic image on the target 5 orients the particles in the suspension and thus permits varying amounts of light from the source 3 to pass through the suspension and be focused upon the viewing screen I5, the energy of the viewed picture being controlled but not generated by the electron beam.

As disclosed in my said original application the suspending medium or liquid I1 contained in the reservoir I3 may be any liquid having the desired characteristics as regards electrical resistance, transparency, vapor pressure and V15 cosity. Thus the suspending medium preferably has very high electrical resistance and transparency and low vapor pressure and viscosity. A number of materials suitable for use as the suspending medium, include liquids such as n-amylsebacate, ethyl-hexyl-phthalate, ethyl-hexylacetate, and tetrabromoethane.

Referring to Figure 3 the curve represents the emission of secondary electrons from the scanned surface of the mica target 5 as ordinates for values of the second anode potential as abscissae.

Line A represents unity secondary electron'emission, a condition which exists when the secondary electron emission from the target is equivalent in quantity to the number of primary electrons impinging on the target. It will be observed that at second anode potentials below the point marked B on the curve the ratio of secondary electrons to primary electrons is less than unity, whereas for second anode potentials between the points B and C on the curve the secondary emission is greater than unity and for higher potentials than that represented by point C this ratio is again less than unity. It may, therefore, be observed that the curve crosses the line A at two points B and C. The points B and C will therefore be referred to as first and second crossover respectively. A target material such as mica has a secondary electron emission characteristic of the general configuration to that shown in Figure 3 and the tube is operated with a potential applied to the anode I exceeding the potential of the second cross-over C. This second cross-over is that potential assumed by an insulated surface when bombarded by an electron beam of such high velocity that for this and all higher velocities of the beam the surface has a secondary emission ratio of less than unity and hence is capable of collecting electrons. I shall, therefore, refer to such a high velocity beam as one which has a volt velocity, that is, velocity in electron volts, greater than the second crossover of the surface scanned. This mode of operation implies that an electrostatic field of controllable magnitude or time of existence or both is applied to the suspending medium in the reservoir I3 at the point where the electron scanning beam strikes or bombards the surface of the target in order to orient the suspended particles in a manner essentially duplicating the light and shades of the original picture. In accordance.

with this principle I accomplish improved operation by scanning the mica target 5 with an electron beam of volt velocity above the second cross-over of the material composing the target 5. I have shown in Figure 3 representative values of volt velocity for purposes of explanation but these values are merely illustrative and are not to be interpreted in a limiting sense. Thus, I have shown the first cross-over B as being at approximately 100 volts, the second cross-over C being at 3000 volts which values are representative of certain materials from which the target 5 may be fabricated such as mica. This is equivalent to saying that if the target 5 is bombarded by electrons having a volt velocity above 3000, such as 5000, the potential of the target under scansion will be 3000 volts. Thus a net potential difference of 2000 volts for the representative values given is produced between the point of the target which is scanned and the electrode l8. It is this potential difference which serves to orient the particles in the suspending medium. For ease of explanation I have previously referred to the formation of an electrostatic image being formed on the target 5 by the variable intensity electron beam. While this image could be measured and its presence actually determined there are other steps involved in the operation which progressively neutralize the electrostatic image. It is desirable to maintain the electrostatic image for a period just short of the time of scanning the target once in the vertical direction so that the particles in the suspending medium are maintained in an oriented condition thereby giving a longer period of illumination on the viewing screen. However, the electrostatic image should be dissipated within a deflnite time period, that is, the charge on the target 5 should be returned to a datum value prior to the next scanning. It is, therefore, necessary to'discharge the electrostatic image and by discharge I mean raisin the potential of the scanned areas from the second cross-overpotential to a. potential approaching that of the anode I, thus in the example given, from 3000 to 5000 volts.

It should be emphasized that the suspension. should be kept at its desired orientation for as long as possible consistent with its readiness to be re-actuated by the next scan, for it is precisely by this persistence effect that the valve offers a considerable improvement over mechanical scanning by a factor equal to the number of picture elements. Thus while the discharge and deorienting mechanisms should not be so slow as to cause undue highlight persistence, neither should they be so fast as to prevent orientation of the particles in the suspension,

or so fast as to darken a scanned element before it is necessary.

I have found that when using volt velocities of the beam below second cross-over, that is, a secondary emission ratio greater than one at a given volt velocity of the beam, not only is the surface being scanned charged up to the potential of anode I by the scanning beam, but secondary electrons liberated from the target at the scanned points tend to charge the scanned points as well as the adjacent areas toward the potential of anode! which serves as a collector of secondary electrons resulting in a detrimental spreading of the charge desired at any point. In my present method of operation, however, when the volt velocity of the beam is above second cross-over for the surface being scanned, adjacent areas while held at a potential approaching that of the anode I are not at the same potential as the scanned areas for the latter are held at the second cross-over potential. Thus if the electrode I8 is held at anode potential, no effective electrostatic field exists across the valve at points other than those scanned and I have found spreading of the effect in the valve to be almost completely eliminated.

In a similar manner, when a beam having a lower volt velocity than the second cross-over volt velocity is used to charge a surface up to anode potential the maximum potential which may be utilized in aligning the particles in the suspending medium is limited by the potential at which the second cross-over occurs. However, in the light valve tube shown in Figure 1 there is no theoretical limitation upon this potential since the potentials of the anode I and of the electrode I8 may be increased as much as desired and the beam will still charge the scanned areas down to 3000 volts, such as for mica.

Furthermore, using a lower volt velocity beam than that represented by the potential of second cross-over, the light valve cannot be actuated by a focused electron beam to produce a fine line of light on the viewing screen l5. This effect is probably due to the action of areas adjacent to the line being scanned being incompletely charged to anode potential thereby constituting a coplanar grid which prevents adequate escape of secondary electrons from this area. If, however, an electron beam having a volt velocity above second cross-over is used, the areas under scansion are charged to second cross-over potential thereby eliminating this co-planar grid effect since areas adjacent to those scanned are at a relatively positive potential rather than at a relatively negative potential. I have found therefore that while employing high velocity electrons having a volt velocity above the second cross-over for the particular target material the valve can be actuated to produce valve action for a very small diameter electron beam whereas with lower velocity electrons it cannot be so actuated. Furthermore under this latter condition the collecting field produced by the anode 1 for collecting secondary electrons from the scanned areas is much improved since the collector is at a considerably higher potential than the surface of the target during scansion. I

It is necessary that the response of the light valve be susceptible of continuous modulation or' graded response in order to permit the reproduction of half-tones. In the operation of my device, this graded response arises from that fact that over an easily determined range of beam current, depending upon the capacity of the valve, the potential below collector to which an area is charged is a reproducible function of the beam current and hence the light passed by the valve is a function of the beam current. Thus I have found that when the trace of the beam is passed rapidly across the target the brightness of the line obtained may be varied smoothly from zero to a maximum by varying the current in the scanning beam.

During the operation of devices such as shown in Figure l I have found that if the electrical resistance of the suspending medium is less than the resistance of the target 5, polarization occurs which may interfere with the desired operation of the device. Thus if the resistance of the suspending medium is too low with respect to the resistance of the target, the electrostatic field across the suspending medium falls to zero too quickly and the condition of orientation of the suspended particles is changed. It may, therefore, be desirable to utilize a target material having the same resistance as that of the suspending medium. In the present device using mica as a material for the target 5 and one of the liquids mentioned above as the suspending medium, this polarization effect is not serious. However, it will be found advantageous to match the resistance of the target material with that of the suspending medium as closely as possible.

In accordance with a further teaching of my invention I may generate additional secondary electrons to more adequately discharge the scanned areas of the mica target directly in the plane of the target without interposing any light absorbing medium between the light source 3 and the target 5, As shown in Figure 4, Imay utilize an electrode structure surrounding and preferably in the plane of the target 5 for the purpose of generating secondary electron emission with which to discharge the target. Referring to Figure 4 which shows only the target 5 and certain associated structure which may be substituted for the target shown in Figure 1, I provide a secondary electron emitting electrode 26 which surrounds the target 5 preferably lying in a plane substantially coplanar therewith. I prefer to form the electrode 26 by depositing a film of metal around the edge or edges of the target 5. The metal may be in suspension and painted thereon to form the electrode 26. This electrode 26 is connected to a somewhat lower potential than that of the anode 1 so that secondary electrons from the electrode 28 may be accelerated to the target I. The electrode 28 should be of a material having a high secondary emission ratio at the particular voltage applied thereto and may be made of a metal such as bright nickel or platinum or of an alloy known to have high secondary electron emission characteristics. The

metal of which the electrode 28 is made, such as bright nickel or platinum, has a considerabhr higher secondary emission second cross-over than the target 5 so that the number of secondary electrons emitted by this electrode when bombarded by the beam from the electron gun will greatly exceed and supplement the number of secondary electrons emitted by the target proper. While I have disclosed the electrode 26 as being of metal, it may be of any material which has a high secondary emission second cross-over or if of material having a second cross-over equivalent to or lower than that of the target proper, it may be operated at a somewhat lower potential than at second anode potential.-

During the operation of the device incorporating a. target such as shown in Figure 4 the target is scanned by the electron beam from the gun through the action of the coils H and V and is also caused to over-scan the target 5 and impinge on the electrode 26 at the beginning and end of each line sweep. Also in accordance with my invention the intensity of the electron beam may be increased during the time or times it impinges upon the electrode 26 so that even greater quantities of secondary electrons may be emitted. This desired action may be obtained by removing or reducing the bias applied to the control electrode of the electron gun 8 at the beginning and end of each line sweep. From the foregoing it will be evident that I have provided a tube or light valve suitable for television reception wherein many disadvantages of the prior art are obviated, and further it will be understood that I am not limited to the specific structure set forth. Thus the electron beam provided for scanning the target may be formed by other structure than the electron gun shown, for example, a beam of high velocity photoelectrons might be utilized. It is likewise obvious that the target may be discharged by means other than those shown in the various modifications of my invention. Therefore, while I have indicated the preferred embodiments of my invention of which I am now aware and have also indicated only one specific application for which m invention may be employed, it will be apparent that my inventionis by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

I claim:

1. An electron discharge device comprising an evacuated envelope having a window portion, said window comprising a sheet of mica hermetically 3. An electron discharge device of the cathode ray type comprising an envelope having a double wall, one 'of said walls being exposed tov the atmosphere within said envelope, said exposed wall comprising mica hermetically sealed to the envelope, a suspension of light intercepting particles between said double wall, an electron gun oppositely disposed from said exposed wall to develop an electron beam, and means to scan said beam over the exposed surface of said mica wall to develop electrostatic charges thereon and to vary the light transmitting properties of said suspension of light intercepting particles.

4. A cathode ray tube for television reception comprising an evacuated envelope, an electron un in said envelope for developing an electron beam of variable intensity, a mica target forming one wall of and sealed hermetically to said envelope whereby one side only of said mica target is exposed to the atmosphere of said envelope, a

light transparent window exposed to and in axial alignment with said mica target, an electrode adapted to liberate secondary electrons adjacent said mica target and exposed to said electron gun, means to scan said beam over said mica target to generate an electrostatic image on said mica target representative of the intensity of said electron beam and over said electrode to develop secondary electrons, a container joined to said envelope having the said mica target as one wall thereof, a window in said container forming another Wall thereof opposite said mica target and in axial alignment with said first mentioned window and a suspension of light intercepting particlesin a liquid carrier filling the space between the walls of said container.

5. A cathode ray tube for television reception including a light transmitting mica target form.

said mica target, a light transmitting electrode.

over one surface of said light transmitting element oppositethe target, a suspension of light intercepting particles in a liquid carrier in said container, an electron gun exposed to said target to generate an electron beam, said gun including means to vary the intensity of said beam, and means to scan the surface of said target opposite that in contact with said suspension with the variable intensity beam of electrons to produce an electrostatic field between said target and said electrode for aligning the light absorbing particles in the suspension.

6. In combination with a light valve for television reception, an evacuated envelope, a target adapted to'be scanned by an electron beam said target forming one wall of said envelope and comprising a sheet of mica hermetically sealed to said envelope to form avacuum tight enclosure.

7. In combination with an electron discharge devicehaving an evacuated envelope, a wall in said envelope consisting of a sheet of mica, and sealing means having a coefficient of expansion closely approaching that oi. the envelope and mica, between the mica sheet and said envelope and hermetically fused to said envelope and said mica.

JOHN S. DONAL, JR. 

